1. Field of the Invention
The present invention relates to a light emitting apparatus having a semiconductor optical amplifier and a grating fiber.
2. Related Background Art
A fiber grating semiconductor laser has a semiconductor optical amplifier and a grating fiber. The semiconductor optical amplifier possesses a front end surface for extraction of light and a rear end surface. This fiber grating semiconductor laser is also provided with an optical cavity consisting of a grating formed in the optical fiber and the rear end surface of the semiconductor optical amplifier. The lasers of this type are known as external cavity type semiconductor laser modules. The emission wavelength of such modules is determined by the Bragg wavelength of the fiber grating.
The inventor has conducted various studies on fabrication of semiconductor laser modules that can be applied in the field of optical communications particularly, in the field of WDM optical communication systems. From these studies the inventor found that the decrease of the bit error rate (BER) tended to be harder in the fiber grating semiconductor laser modules than in the DFB semiconductor lasers.
In the WDM optical communication systems, the bit error rate to be achieved should be equal to or less 10xe2x88x9210 and preferably 10xe2x88x9212. According to the studies by the inventor, it was, however, impossible to reach the above target even in measurement in the laboratory. According to the data presented in documents, the bit error rate becomes saturated around 10xe2x88x9210. A document describing it is, for example, Document (ECOC 97, Sep. 22-25, 1997, Conference Publication No. 448, xe2x80x9cDENSE WDM TRANSMISSION IN STANDARD FIBRE USING DIRECTLY-MODULATED FIBRE GRATING LASERS AT 2.6 GBIT/Sxe2x80x9d). The above BER value was considered heretofore as a limit.
It is an object of the present invention to provide a light emitting apparatus that can offer characteristics over the bit error rate of the conventional limit.
In order to accomplish this object, the inventor has performed further studies. The inventor first made a comparison between the fiber grating semiconductor lasers and the DFB semiconductor lasers.
According to the comparison, the following points are common to the fiber grating semiconductor lasers and the DFB semiconductor lasers: (a) a semiconductor element is used as a light generating source; (b) they include the optical cavity consisting of the diffraction grating and one end surface of the semiconductor element. In the DFB semiconductor lasers, however, the feedback mechanism for optical amplification is provided by the grating formed in the semiconductor element. In the fiber grating semiconductor lasers, the optical cavity is composed of the rear end surface of the semiconductor element and the grating formed in the grating fiber on the other hand. In other words, the fiber grating semiconductor lasers have such a structural feature that the grating is provided outside the semiconductor element.
The inventor has done further studies on this structural difference. From the studies, the inventor realized that the cavity length of the fiber grating semiconductor lasers was longer than that of the DFB semiconductor lasers. This is a big difference from the DFB semiconductor lasers.
The semiconductor lasers with such longer cavity length demonstrate narrower oscillation spectral width. The inventor noted that the narrower spectral width meant weakness against disturbance from the outside. This means that, with the external disturbance on the fiber grating semiconductor lasers, the disturbance affects the oscillation wavelength thereof more than that on the DFB semiconductor lasers.
Then the inventor studied the disturbance on the fiber grating semiconductor lasers. Since the disturbance related to the phenomenon of bit error rate in very fast optical transmission, for example, the phenomenon of about 2.5 Gbit/sec, the environmental factor of temperature was excluded from the studies. The inventor noted electric disturbance and optical disturbance.
First, the inventor studied the electric disturbance. The DFB semiconductor lasers and the fiber grating semiconductor lasers both are installed in similar packages, e.g., in butterfly type packages. Although the fiber grating semiconductor lasers incorporate the semiconductor optical amplifier, the semiconductor optical amplifier has the structure very similar to that of the DFB semiconductor lasers. For this reason, the inventor considered that the impedance of power-supply wires and signal wires in the fiber grating semiconductor lasers should also be close to that in the DFB semiconductor lasers. Therefore, the inventor concluded that there existed no difference associated with the increase of bit error rate between these lasers in the process of converting electric current to light.
Next, the inventor studied the optical disturbance. Since the external disturbance is associated with very high frequencies, a light signal generated by itself can be a source of disturbance. Optical feedback or return light of the light signal is added through the grating fiber to the semiconductor optical amplifier. If this return light is the cause, it will not contradict with the structural factor that the fiber grating semiconductor lasers have the longer cavity length than the DFB semiconductor lasers.
It is, however, common practice to place an optical isolator on the path of the return light. Then the inventor carried out experiments for further detailed studies on the return light. From these experiments, the inventor has discovered that the return light degrades the bit error rate and that a required isolation value relates to the bit error rate to be achieved.
Under such circumstances, the inventor accomplished the present invention as follows.
A light emitting apparatus according to one aspect of the present invention comprises a light generating portion and an optical isolator. The light generating portion includes a semiconductor optical amplifier and a grating fiber. The semiconductor optical amplifier has a light emitting surface and a light reflecting surface. The grating fiber has an optical waveguide and a grating provided in the optical waveguide. The optical waveguide has a first end and a second end. The first end of the optical waveguide is optically coupled to the light emitting surface of the semiconductor optical amplifier. The optical isolator is provided between the light generating portion and an output of the light emitting apparatus. The isolation value of the optical isolator is not less than a value specified by the following:
xe2x88x9252.4xe2x88x928.7xc3x97log(BER),
where the isolation value is expressed in dB with respect to the bit error rate BER to be achieved at the transmission rate of 2.5 Gbps in the 1.55-xcexcm band.
A light emitting apparatus according to another aspect of the present invention comprises a plurality of (n) light generating portions and an optical isolator. Here n is a natural number not less than 2. Each of the plurality of (n) light generating portions includes a semiconductor optical amplifier and a grating fiber. The semiconductor optical amplifier has a light emitting surface and a light reflecting surface. The grating fiber has an optical waveguide and a diffraction grating provided in the optical waveguide. The optical waveguide has a first end and a second end. The first end of each optical waveguide is optically coupled to the light emitting surface of the corresponding semiconductor optical amplifier. The optical isolator is provided between the plurality (n) of light generating portions and an output of the light emitting apparatus.
In this light emitting apparatus, a star coupler is provided between the plurality of (n) light generating portions and the optical isolator. The isolation of the optical isolator is not less than a value specified by the following:
xe2x88x9252.4xe2x88x928.7xc3x97log(BER)xe2x88x9210xc3x97log(n),
where the isolation is expressed in dB with respect to the bit error rate BER to be achieved at the transmission rate of 2.5 Gbps in the 1.55-xcexcm band.
In this light emitting apparatus an AWG (Arrayed Wave Guide) is provided between the plurality of (n) light generating portions and the optical isolator. The isolation of the optical isolator is not less than a value specified by the following:
xe2x88x9252.4xe2x88x928.7xc3x97log(BER)xe2x88x922xc3x97xcex1,
where xcex1=5 dB, and the isolation is expressed in dB with respect to the bit error rate BER to be achieved at the transmission rate of 2.5 Gbps in the 1.55-xcexcm band.